Please help my brain hurts

I have been trying to understand apertures. As I understand it a f1 would mean completely open f1.4 half of that f2 half again and so on. Yet does the actual size eg diameter of the front element have any effect on the actual light gaining ability of the lens. For example a f2 rangefinder lens vs a f2 slr lens may have different size front elements, does this then mean that the slr lens would be better for low light f stops being equal, or does the flange distance negate such benefits?

Does the sizes of the front element correlate proportionate to the sensor size?